1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of performing uplink synchronization through a random access procedure in a wireless communication system.
2. Discussion of the Background Art
Third generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment (UE), and the like are defined as requirements.
In general, there are one or more cells within the coverage of a base station (BS). One cell may include a plurality of UEs. A UE generally performs a random access procedure to access a network. The random access procedure is performed for various purposes, for example, for uplink synchronization between the UE and the network, uplink radio resource allocation request, etc.
The random access procedure starts when the UE transmits a random access preamble. The UE randomly selects one random access preamble from predetermined 64 random access preambles, and transmits the selected random access preamble. Although one preamble is randomly selected from 64 random access preambles, two or more UE can simultaneously perform the random access procedure by using the same random access preamble. This is called a contention-based random access procedure. On the contrary, when a dedicated random access preamble is assigned to each UE, it is called a non-contention based random access procedure.
One of the purposes of performing the random access procedure is to perform uplink synchronization. When collision occurs between UEs in the contention based random access procedure, whether collision occurs or not cannot be known until contention resolution is achieved between the BS and the UE. Therefore, even if collision occurs, the UE may perform uplink synchronization by using time alignment information transmitted from the BS, resulting in incorrect synchronization. The incorrect uplink synchronization performed by the UE may act as interference to another UE, which may lead to deterioration in reliability of a communication system.